Group A streptococci (GAS; Streptococcus pyogenes) are highly prevalent bacterial pathogens that infect the throat or skin of humans, causing pharyngitis or impetigo. GAS can trigger autoimmune disease, such as rheumatic fever, or cause life-threatening infections, such as toxic shock syndrome. Macrolide-resistance in GAS developed abruptly in the 1980s and accelerated in the 1990s, exceeding 30% of GAS isolates in some regions. The long-term goal of the proposed study is to understand the molecular mechanisms underlying the emergence, spread, and persistence of macrolideo resistance in GAS. The proposed research combines epidemiology, molecular genetics, and basic evolutionary principles in novel ways. Aim 1 will seek to define the molecular characteristics of macrolide-resistant clones of GAS collected from throughout the world. Using nucleotide sequence determination, isolates will be defined for emm-type, resistant genes and housekeeping loci, via multilocus sequence typing. A database will be maintained on the Internet, to facilitate worldwide surveillance. Aim 2 will seek to trace the recent evolutionary history of newly emerged resistant clones. Evolutionary models can be used to address several novel issues: (a) estimate the number of times resistant strains arose, (b) evaluate the significance of immune escape in the spread of resistance genes, (c) uncover bias in the order of acquisition of multiple resistance genes, and (d) identify problematic clones and lineages based on their evolutionary trajectory. By tracking nucleotide polymorphisms arising via recombination, a new molecular archaeological approach will also be developed, to predict which part(s) of the world resistance most likely emerged, and the geographical routes of clonal spread. Future studies can complete the evolutionary framework for all macrolide-resistant clones, extend the questions on genetic dynamics (a-d, above), and assess differences for efflux versus methylase resistance mechanisms. The high global prevalence of GAS, combined with its high rate of genetic recombination, makes it ripe for quickly evolving in unexpected ways. GAS provide a sound model system for understanding the molecular evolution underlying the early stages of the emergence and global spread of antibiotic resistance.